FIGS. 1A and 1B illustrate prior-art implementations of alkali-metal thermal-to-electric converters (AMTECs), 100 and 120. In the implementation of FIG. 1A, a plenum 101 is divided into high and low sodium vapor pressure chambers 102, 103 with test-tube-shaped thermal-to-electric cells 105 projecting from the high pressure chamber into the low pressure chamber. Each of the cells is formed by a solid electrolyte projection 107 sandwiched between porous interior and exterior electrodes—the anode 109 and cathode 111, respectively. The solid electrolyte serves as both an ionic conductor and an electronic barrier, conducting heat-activated sodium ions between the two chambers (i.e., across the pressure gradient) while the corresponding free electrons are collected at the anode. The resulting electric potential across the solid electrolyte yields electronic current from the anode through an external load 115 (delivering power) and back to the cathode where the electrons recombine with the electrolyte-crossing sodium ions in a reduction (or neutralization) to sodium vapor. An electromagnetic pump (not shown) is provided to return pooled liquid metal sodium from the low pressure chamber to the high pressure chamber.
To limit power-draining ohmic losses, wire-wrapping or other auxiliary current collection structures typically overlay the relatively low-conductance porous electrodes. Unfortunately, such structures tend to degrade prematurely in the thermally challenging environment of the converter. For example, wrapped wires tend to lose physical and electrical contact over time (e.g., due to non-uniform thermal expansion/contraction of the wires and structures they encircle), increasing I2R loss and thus degrading the power density of the converter.
In thermal-to-electric converter 120, shown in cross-section in FIG. 1B, a stack of series-coupled, ring-shaped AMTEC cells 1251-1253 extends between the upper and lower walls of a cylindrical plenum 122, thus forming concentric annular chambers 122, 123 (i.e., high and low vapor pressure chambers) separated by the solid electrolyte cell walls. In theory, this arrangement mitigates the I2R losses that plague the tubular cell design by raising the output voltage (i.e., the sum of the voltages of individual cells by virtue of their series interconnection) and correspondingly lowering the output current of the overall converter. In practice, however, this coaxial chamber arrangement suffers from a number of drawbacks. First, care must be taken to avoid shorting the anodes or cathodes of adjacent cells (i.e., electrodes at different potentials) through the cell-to-cell interconnect structure, a complication addressed by leaving gap 131 (shown in detail view 130) between the anode of each lower potential cell and the interconnect to the higher potential cell below (a similar gap 132 is provided between the cathode of each cell and the interconnect to the lower potential cell above). Unfortunately, unless these electrode gaps, which constitute dead zones from a power generation standpoint, are made impractically large, significant leakage current flows across the voltage gradient, degrading device power density. Also, the junction of the exposed solid electrolyte and conductive interconnect in the presence of hot sodium vapor constitutes a triple point at which electrons released by sodium ionizations are shunted through the cell-to-cell interconnect to the cell cathode, further depleting the voltage differential across the solid electrolyte and thus further degrading the power density of the converter. While sputtered non-porous insulators have been proposed to cover the gap (and thus the triple point), the viability of such coverings are doubted, particularly in the high-pressure chamber. Further, investigations show that sputtered coatings do little to reduce leakage current flowing across the voltage gradient between anode and interconnect.
The annular design of converter 120 brings additional complications. For one, the cell stack and its interconnection to opposite ends of the plenum housing are subjected to significant stress/strain during thermal expansion/contraction (i.e., as the plenum housing and cell stack components tend to exhibit expand/contract non-uniformly), mechanical wear forces that tend to degrade device power density and lead to premature failure, particularly in applications that involve frequent temperature cycling. The coaxial heating arrangement also adds complexity (requiring heat to be injected into a blind hole) and tends to be thermally inefficient as heat radiates directly from the interior heat source toward the cold containment wall of the plenum. Perhaps more significantly, the large temperature gradient between the heat source and plenum wall (and relatively short distance between the cell stack and cold plenum wall) and makes it difficult to prevent sodium condensation on the cell stack surface, a highly problematic phenomenon as the electrically conductive sodium condensate can short the different-potential cells to one another, severely disrupting operation of the converter.
Yet other issues plague the implementation of FIG. 1B. For example, the high voltage across the gap between the final cell in the stack and its interconnection to the housing (i.e., in view of the physical cell stack connection at both ends of the housing) presents a source of leakage current that becomes increasingly troublesome as the cell count, and thus the voltage across the last gap, grows, discouraging more than a relatively small number of cells in the complete stack (e.g., three cells). Similarly the metal plates 135 used to interconnect adjacent cells not only increase component count, but also constitute dead zones that consume an increasing proportion of the cell stack surface area as cell count grows, again discouraging more than a small number of stacked cells. Also, in the implementations of both FIGS. 1A and 1B, the sensitive device cathode is exposed by line-of-sight contact to the chamber walls of the low-pressure vapor chamber, leading to life-shortening electrode contamination from materials in the housing, such as Chromium or Manganese escaping from stainless steel.